1. Field of the Invention
The invention relates to a mobile station in a CDMA wireless communication system, a method of controlling power for transmitting a CDMA wireless signal in a wireless communication system, an apparatus for controlling power for transmitting a wireless signal in a CDMA wireless communication system, and a computer readable medium encoded with program for causing a computer to carry out a method of controlling power for transmitting a wireless signal in a CDMA wireless communication system. The invention relates more particularly to signal-transmission power control in a situation in which a mobile station makes communication with a plurality of base stations.
2. Description of the Related Art
A wireless communication terminal such as a mobile phone, PHS (personal handy phone system), PDA (personal digital assistant), a pager, and a device capable of making communication with a wireless LAN is now popularized, and further expansion of a service area and variation in services are expected.
Wireless communication technology may be grouped into TDMA (Time Division Multiple Access) and CDMA (Code Division Multiple Access), for instance. Hereinbelow is explained CDMA to which the present invention is preferably applied.
In a CDMA wireless communication system, communication is made as follows.
First, a transmitter spreads signals with pseudorandom noise (PN) codes of the transmitter, and then, transmits the signals.
On receipt of the signals from the transmitter, a receiver de-spreads the received signals with the same pseudorandom noise (PN) codes as those of the transmitter to reproduce the signals transmitted from the transmitter.
In CDMA, a plurality of transmitters uses pseudorandom noise (PN) codes perpendicular to and different from one another, and hence, it is possible to differentiate the transmitters from one another. Thus, a plurality of transmitters can transmit wireless signals having a common frequency.
However, it is quite difficult to render all pseudorandom noise (PN) codes perpendicular to one another. Actually, pseudorandom noise (PN) codes are not completely perpendicular to one another, that is, there exists correlation among pseudorandom noise (PN) codes. Such correlation interferes with communication, resulting in degradation in communication quality. Since correlation causes interference, as a number of transmitters increases, interference increases.
In a wireless communication system, communication is not made directly between mobile stations corresponding to wireless communication terminals, but made through a base station. Hence, there is caused a problem that signals transmitted from a mobile station located in the vicinity of a base station interfere with weak signals transmitted from a mobile station located far away from the base station. In a CDMA wireless communication system, in order to avoid degradation in communication quality caused by the above-mentioned problem, power by which a wireless signal is transmitted to a mobile station (hereinafter, referred to as “signal-transmission power”) is controlled.
Hereinbelow is explained an example in which signal-transmission control is applied to W-CDMA (Wideband-CDMA) which is one of IMT-2000.
A base station controls signal-transmission power such that Eb/I0 (a ratio of desired power (Eb) for receiving a signal to power (I0) of an interference signal) obtained through communication channel established to each of mobile stations is kept equal to a predetermined Eb/I0 in order to ensure requisite communication quality. Specifically, a base station controls signal-transmission power as follows.
A base station periodically transmits a down-channel signal to a mobile station together with TPC (Transmission Power Control) data in accordance with which power by which a mobile station transmits a signal to a base station is increased or reduced.
If SIR (Signal to Interference Ratio) of a signal received from a mobile station is smaller than Eb/I0, a base station transmits TPC data to the mobile station to increase the power. In contrast, if SIR of a signal received from a mobile station is greater than Eb/I0, a base station transmits TPC data to the mobile station to reduce the power. Herein, SIR of a signal received from a mobile station is used as one of data indicative of wireless communication quality.
The mobile station controls the power by which an up-channel signal is transmitted to the base station, in accordance with the received TPC data.
The above-mentioned transmission power control is described in “3GPP TS 25.214 V3. 12. 0”, pp. 11-21, Apr. 4, 2003, 3GPP (printed on Jul. 7, 2004 through Internet from the site having URL “http://www.3gpp.org/ftp/Specs/html-info/25214.htm).
Transmission power control is carried out to power for transmitting a signal through down-channel.
A mobile station transmits an upward signal to a base station together with TPC data in accordance with which power for transmitting a signal to the mobile station from the base station is increased or reduced. The base station controls power for transmitting a signal to the mobile station, in accordance with the TPC data received from the mobile station.
The above-mentioned transmission power control is described in “3GPP TS 25.214 V3. 12. 0”, pp. 21-27, Apr. 4, 2003, 3GPP (printed on Jul. 7, 2004 through Internet from the site having URL “http://www.3gpp.org/ftp/Specs/html-info/25214.htm).
In a wireless communication system, in which each of base stations transmits signals having different frequencies from one another, it is necessary to switch a frequency each time of handover, and hence, communication is instantaneously interrupted. Herein, handover means a function of a mobile station for automatically switch a base station to continue communication, when the mobile station moves into a second service area of a second base station from a first service area of a first base station.
In contrast, since it is possible for base stations in a W-CDMA wireless communication system to use signals having a common frequency, it is not necessary for a mobile station to switch a current frequency to a frequency associated with each of base stations at handover. Accordingly, a W-CDMA wireless communication system allows a mobile station to carry out soft handover (SHO), that is, to carry out handover without instantaneous interruption of communication, as described in “Spectrum Spreading Technology”, Kenichi MATUO, Tokyo Denki University Publishing section, May 30, 2002, pp. 230-231.
Herein, soft handover (SHO) is defined as handover to be carried out keeping a mobile station in communication with a plurality of adjacent base stations through the use of signals having a common frequency, and is inherent to a W-CDMA wireless communication system.
In a W-CDMA wireless communication system, power for transmitting upward and downward signals between a mobile station and a base station is controlled even during SHO. When power for transmitting a downward signal is to be controlled, a mobile station transmits common TPC data to a plurality of base stations with which the mobile station is in communication.
When power for transmitting an upward signal is to be controlled, a mobile station controls the power in accordance with a plurality of TPC data received from a plurality of base stations with which the mobile station is in communication. However, since a mobile station is usually designed to include a single signal-transmission circuit, it is necessary to unite a plurality of TPC data into single TPC data.
A plurality of TPC data can be degenerated into single TPC data in many ways. The standard specification defines Algorithm 1 and Algorithm 2 (see 3GPP TS 25.214 V3. 12. 0”, p. 11-21).
In a system in which transmission power control is carried out between a plurality of base stations and a single mobile station, even if a part of the base stations transmits TPC data to the mobile station to increase power by which the mobile station transmits a signal, and the mobile station increases the power in response to the received TPC data, communication quality might not meet desired requirement.
Various reasons are considered to the above-mentioned problem. For instance, in an environment in which complicated phasing occurs, a bit error rate (BER) would not be improved, even if the power is increases, due to the phasing, resulting in that communication quality does not meet desired requirement. However, since a base station receives a signal which does not meet the required communication quality, the base station keeps transmitting TPC data to the mobile station to increase power by which the mobile station transmits a signal to the base station.
When a mobile station is located remarkably far away from a base station, the base station keeps transmitting TPC data to the mobile station to increase power by which the mobile station transmits a signal to the base station.
Furthermore, when a base station is out of order, the base station often keeps transmitting TPC data to the mobile station to increase power by which the mobile station transmits a signal to the base station.
If a certain base station keeps transmitting TPC data to a mobile station to increase power by which the mobile station transmits a signal to the base station, the power is increased excessively, and resultingly, the mobile station may transmit a signal to the base station with higher power than necessary.
If a certain base station transmits a signal with higher power than necessary, interference would be unpreferably increased to other base stations. Furthermore, increase in interference would cause reduction in communication channel capacity of a base station.
Transmission power control during SHO has been suggested as follows.
For instance, WO99/37111 has suggested transmission power control to be carried out to a mobile station during SHO in a CDMA wireless communication system, in order to optimally control power by which a mobile station transmits a signal during SHO. On receipt of various TPC data from a plurality of base stations during SHO, a mobile station calculates reliability data to apply weighting to TPC data in proportion to transmission power, but in inverse proportion to noise power, based on both signal power obtained by averaging the received TPC data with a predetermined time constant, and an average SIR in a recent period of time. The mobile station weights the received TPC data with the reliability data, and controls power by which a signal is transmitted to the base station, in accordance with the thus weighted TPC data.
Though the TPC data is averaged with a predetermined time constant, the transmission power control suggested in WO99/37111 is accompanied with a problem that it is not possible to avoid a mobile station from being influenced by a base station keeping transmitting TPC data to increase power by which the mobile station transmits a signal to the base station.
Japanese Patent Application Publication No. 2001-211119 has suggested transmission power control in a CDMA wireless communication system in order to reduce an error in transmission power control by varying a step size in accordance with which transmission power is controlled, when control delay exists. The transmission power control is carried out based on a known relation between statistical characteristics of TPC commands and an optimal step size. Specifically, an optical step size is determined statistically based on stored TPC commands.
However the suggested transmission power control is accompanied with a problem that it is not possible to avoid a mobile station from being influenced by a base station keeping transmitting TPC data to increase power by which the mobile station transmits a signal to the base station.
Japanese Patent Application Publication No. 2002-217751 has suggested a transmission power controller including a first unit which compensates for a symbol point of data in accordance with a corrected amplitude, and a second unit which, based on a first signal indicative of whether it is necessary to compensate for an amplitude of a symbol, and a second signal indicative of masking at least one of symbol point components, compensates for an amplitude of the symbol, and transmits the thus compensated amplitude to the first unit.
Japanese Patent Application Publication No. 2003-87184 has suggested a method of judging a condition of an electric field in a mobile station in a mobile communication system, including the steps of counting a number of TPC bits in a plurality of slots in a predetermined period of time, measuring an intensity of an electric field of a signal received from a base station, calculating TPC bit estimate, based on the number of TPC bits, calculating field intensity estimate, based on the measured intensity of the electric field, calculating field condition estimate, based on the TPC bit estimate and the field intensity estimate, and judging a condition of the electric field, based on the field condition estimate.
Japanese Patent Application Publication No. 2003-204299 has suggested a control system for controlling a velocity of a data signal transmitted to a mobile station through down-shared channel in a mobile packet communication system. The control system includes a first unit which receives transmission power control data of downward channel transmitted through upward channel, a second unit which stores therein the transmission power control data of downward channel, and a third unit which controls a velocity of a data signal transmitted to a mobile station through down-shared channel, in accordance with the transmission power control data of downward channel stored in the second unit.
Japanese Patent Application Publication No. 2004-88333 has suggested a transmission power controller including a first unit which judges whether received power is equal to or greater than a threshold, a second unit which judges whether transmission power control data included in received data indicates “increase” or “reduction”, and a third unit. The second unit transmits the judgment as it is to the third unit, if the received power is equal to or greater than the threshold, or instructs the third unit to increase power, if the received power is smaller than the threshold, and further if the transmission power control data successively twice indicates “increase”. The third unit increases or reduces power in accordance with the instruction received from the second unit.